Such methods for ascertaining and monitoring fill level in a container are frequently applied in measuring devices of automation and process control technology. Fill level measuring devices of this type are available from Endress+Hauser, for example, under the marks PROSONIC, LEVELFLEX AND MICROPILOT. These work on the basis of the travel time, measuring method and serve to determine and/or to monitor a fill level of a medium in a container. These fill level measuring devices transmit a periodic transmission signal in the microwave, or ultrasonic, domain by means of a transmitting/receiving element in the direction of the surface of a fill substance and receive the reflected echo signals following a distance dependent, travel time. Usual fill level measuring devices working with microwaves can be divided, basically, into two classes: a first class, in the case of which the microwaves are sent by means of an antenna in the direction of the fill substance, reflected on the surface of the fill substance and then, after a distance dependent, travel time, are received back; and a second class, in the case of which the microwaves are guided along a waveguide in the direction of the fill substance, are reflected on the surface of the fill substance on the basis of the impedance jump existing there, and the reflected waves are then guided back along the waveguide.
From the received echo signals, as a rule, an echo function representing the echo amplitudes as a function of the travel time is formed, wherein each value of this echo function corresponds to the amplitude of an echo reflected at a certain distance from the transmission element.
In this ascertained echo function, a wanted echo is determined, which corresponds to the reflection of the transmission signal on the surface of the fill substance. From the travel time of the wanted echo, one can, in the case of known propagation velocity of the transmission signals, directly ascertain the distance between transmission element and the surface of the fill substance.
In order to simplify the echo curve evaluation, it is not the received, raw signal of the pulse sequence, which is used, but, instead, the so-called envelope curve. The envelope curve is obtained, for example, by rectifying the raw signal of the pulse sequence and then filtering via a lowpass.
There are a number of different methods for detecting the wanted echo in an envelope curve, and these can be divided basically into two groups: either the static ascertainment methods with static echo search algorithms; and/or the dynamic ascertainment methods with dynamic echo search algorithms.
In a first method of the static echo search group, using a static echo search algorithm, that echo is selected as a wanted echo, which has a larger amplitude than the remaining echos. It is, thus, the echo with the largest amplitude in the envelope curve, which is selected as the wanted echo.
In a second method of the static echo search group, it is assumed by a static echo search algorithm, that the wanted echo is the first echo in the envelope curve after the transmission pulse. Thus, the first echo in the envelope curve is selected as the wanted echo.
It is possible to combine the two methods with one another in a static echo search algorithm, by defining e.g. a so-called, first echo factor. The first echo factor is a predetermined factor, by which an echo must exceed a certain amplitude, in order to be recognized as a wanted echo. Alternatively, a travel time dependent echo threshold can be defined, which an echo must exceed, in order to be recognized as the wanted echo.
In a third method, the fill level, measuring device is told, once, the current fill level. The fill level, measuring device can, on the basis of such input fill level, identify the associated echo as the wanted echo and e.g. track it by a suitable, dynamic, echo search algorithm. Such methods are referred to as echo tracking methods. In such case, e.g. in each measuring cycle, maxima of the echo signal or the echo function are ascertained and, on the basis of the knowledge of the fill level ascertained in the preceding measuring cycle and an application-specific, maximum expected rate of change of the fill level, the wanted echo is detected. From the travel time of the so-ascertained, current wanted echo, there results, then, the new fill level.
A fourth method is described in DE 102 60 962 A1. There, the wanted echo is ascertained on the basis of data stored earlier in a memory. In such case, echo functions are derived from received echo signals. The echo functions show the amplitudes of the echo signals as a function of travel time. The echo functions are stored in a table, wherein each column serves for accommodating an echo function. The echo functions are stored in the columns in a sequence, which corresponds to the fill level associated with the respective echo functions. In operation, the wanted echo and the associated fill level are ascertained with the assistance of the table on the basis of the echo function of the current transmission signal.
In DE 103 60 710 A1, a fifth method is described, wherein, periodically, transmission signals are sent in the direction of the fill substance, their echo signals recorded and converted into an echo function, at least one echo characteristic of the echo function is ascertained, and, on the basis of the echo characteristics of at least one preceding measuring, a prediction for the echo characteristics to be expected in the case of the current measuring is derived. The echo characteristics of the current measuring are ascertained, taking into consideration the prediction, and, on the basis of the echo characteristics, the current fill level is ascertained. This method comes closest to being an echo tracking.
In DE 10 2004 052 110 A1, a sixth method is described for improving wanted echo detection through echo evaluation and classification of the echos in the envelope curve.
The above described methods work per se in a plurality of applications without problem. Problems occur, however, always when the echo stemming from the fill level on the basis of the method cannot be identified without there being some doubt as to the correctness of the identification.
In the case of the first method, there occurs, for example, measurement problems, when installed objects are present in the container, which reflect the transmission signals better than the surface of the fill substance.
In the case of echo tracking according to the third method, measurement problems occur, when, during operation, the wanted echo runs together with a disturbance echo and, subsequently, the disturbance echo is tracked as an incorrect wanted echo. Furthermore, a problem occurs, when, upon turn-on, the preceding wanted echo signal no longer fits the actual situation, or the preceding wanted echo signal is not known.
If, mistakenly, another echo than the fill level echo is classified as a wanted echo, there is the danger that a wrong fill level is output, without such being noticed. This can, depending on application, lead to an overfilling of containers, to operation of pumps empty or to other events connected, on occasion, with considerable danger.